For large electric subsea consumers, typically electrical motors for hydrocarbon compression or hydrocarbon pumps, it is known to have a subsea electric control system to monitor their function and various associated parameters. Additionally, other subsea low voltage/low power auxiliary systems may be present.
For some known systems, such as systems with topside VSDs (variable speed drives) and long cables with high voltage transmission via subsea cables to subsea compressors/pumps, such control power supply is provided with a topside UPS (uninterruptable power supply) that connects to the subsea control system. However, with long step-outs, typically above 50-60 km, the Ferranti effect in the control power supply cable results in stability challenges for the subsea CPDU (control power distribution unit) and the topside UPS due to reactive power flow from capacitance in the long cable. Thus, for long step-outs it is appropriate to use a subsea UPS unit instead of a topside UPS, and to ensure that the subsea UPS input voltage and frequency is within an acceptable range. For instance, one may use one or two subsea UPS units per compressor train (each train typically includes one compressor and one pump). There are also cases where there may be only compressors or only pumps on the seabed, where the same type of control system power supply applies.
FIG. 1 illustrates a setup according to the prior art, where two UPS's arranged subsea are provided with power through a dedicated umbilical. This will be described in detail further below.
Normally at least two UPS units are used subsea for redundant control supply, which is a necessity for safe operation of the control system in case control power is lost or the subsea boosting system is tripped. In such a case of loss of power, it is crucial to have UPS units that can provide enough power for safe shutdown and sleep mode of the subsea control system. To supply the subsea UPS units on the seabed with power, the obvious solution is to arrange a dedicated power supply line to the subsea UPS, either in a separate umbilical or as an integrated part of the power umbilical/cables supplying the subsea motors for the compressors and/or pumps. However, for long step-outs, that would dramatically increase cost due to more cables and elements in the umbilical(s). The most relevant solution when combining control power and motor power in the same power umbilical would be to arrange 3 more conductors in the umbilical that delivers power to the motor. Typically, one would use one umbilical per compressor train. There is however a problem in that the number of supply systems (multiples of 3 cables (conductors/phases) per 3-phase power supply system) per umbilical is limited due to risk of cross-talk and the number of layers in the umbilical before it gets too large in diameter or cost gets too high.
For the cases with compressor train power supply, the umbilical normally has the compressor motor power supply in the center, and pump motor supply cables are twisted in the layer about the center portion (cf. FIG. 2a). Twisting is used to electromagnetically de-couple the 3-phase systems/layers. This eliminates induced voltages and potential torque pulsations. This is in order to make sure that there is no crosstalk from the compressor power supply conductors to the pump power supply conductors (the same logic applies if there are only pumps or only compressor on the seabed). However, having three more conductors for the subsea UPS power supply in the same umbilical would involve technical challenges or disadvantages. Such conductors could be arranged in a third outer layer (cf. FIG. 2c), however that would result in an excessive outer diameter of the umbilical. Alternatively, they could be arranged in the second and thus the same layer as the pump power supply conductors (cf. FIG. 2b). However, when conductors are in the same layer, there will be crosstalk. There is then a risk that the pump power supply (in general the 3-phase system in the same umbilical layer as the control power supply) will give unwanted crosstalk into the control system power supply, and vice versa, for instance during a short-circuit.
Hence, an objective of the present invention is to find a solution of supplying electric power to remotely arranged subsea UPS units that avoids such problems.
Publications WO2007055593 and WO2007055594 describe a subsea power supply assembly in which electric power of constant frequency is delivered from a topside main power source to a subsea location. At the subsea location, power is distributed to auxiliary power consumers (e.g. control power) and subsea variable speed drives which delivers power to electric motors. In such a setup, a vast amount of equipment needs to be installed at the subsea location.
A similar setup is described in WO2007055587, where a UPS system for subsea use is discussed. Also in this setup, variable speed drives are installed subsea for delivering power of variable frequency to the large electric consumers, namely the electric motors for pumps and compressors.
European patent application EP1316672 describes a power supply system where power is transmitted from topside to the subsea location with a high voltage DC cable.